The disclosure generally relates to a fuel vapor processing apparatus for processing fuel vapor that may be generated in a fuel tank.
A known fuel vapor processing apparatus may include a canister for adsorbing fuel vapor generated in a fuel tank, an atmospheric passage for communicating the canister with the atmosphere, a vapor passage for communicating between the fuel tank and the canister, a purge passage for communicating between the canister and an intake pipe that may supply air to an internal combustion engine, a negative pressure applying device that may apply a negative pressure to the canister for adsorbing fuel vapor from within the canister, and a pressure regulating device for regulating the negative pressure applied by the negative pressure applying device. Fuel vapor generated in the fuel tank may flow from within the fuel tank to the canister via the vapor passage, so that the canister can adsorb the fuel vapor. For desorbing the adsorbed fuel vapor from within the canister, the negative pressure applying device may apply the negative pressure to the canister, so that the fuel vapor may be desorbed from within the canister and may then be supplied to the intake pipe via the purge passage.
This type of fuel vapor processing apparatus is disclosed, for example, in Japanese Laid-Open Patent Publication No. 2002-188530. In this publication, a vacuum pump is disposed in the purge passage and may serve as the negative pressure applying device. An electromagnetic valve is also disposed in the purge passage and may serve as the pressure regulating device. The electromagnetic valve may be controlled with respect to a duty ratio by an engine control unit (ECU) of a vehicle engine, so that an open ratio of the vapor passage may be controlled. The electromagnetic valve may be disposed in a portion of the purge passage between the vacuum pump (negative pressure applying device) and the intake valve. Thus, in this publication, the pressure regulating device is disposed on a downstream side of the pressure applying device with respect to a direction of flow of a purge gas (i.e., a mixture of air and fuel vapor) when the fuel vapor is desorbed from the canister and purged to the engine. The direction of flow of the purge gas will be hereinafter called a “purge direction.”
When the negative pressure is applied to the canister by the negative pressure applying device, the purge gas may forcibly flow toward the downstream side (i.e., toward the intake pipe) of the negative pressure applying device. Because the pressure adjusting device is located on the downstream side of the negative pressure applying device with respect to the purge direction, in some cases, it may be possible that a positive pressure is applied to the upstream side of the negative pressure applying device with respect to the purge direction. Such application of the positive pressure may be caused by the following first to third reasons. The first reason is a pressure loss caused by a piping resistance against flow of the gas, for example, in the pressure regulating device or the purge passage. The second reason is a difference between the time when the operation of the pressure regulating device is stopped and the time when the operation of the negative pressure applying device is stopped. The third reasons is a difference between a set pressure of the pressure regulating device and the pressure applied by the negative pressure applying device.
Regarding the second reason, if the electromagnetic valve is used as the pressure regulating device, the electromagnetic valve may immediately close upon receipt of a stop signal from the ECU. On the other hand, due to an inertia force, the vacuum pump as the negative pressure applying device may not immediately stop its operation upon receipt of a stop signal from the ECU. Therefore, even in the case that the pressure regulating device and the negative pressure applying device have simultaneously received the stop signals from the ECU, it may be possible that the vacuum pump still operates due to the inertia force after the electromagnetic valve has stopped. If this occurs, a pressure in a portion of the purge passage between the electromagnetic valve and the vacuum pump may gradually increase. Regarding the third reason, if the set pressure of the pressure adjusting device is smaller than the pressure applied by the negative pressure applying device, a pressure in a portion of the purge passage between the electromagnetic valve and the vacuum pump may also gradually increase. This is because an amount of flow of the gas flowing through the pressure regulating device may be smaller than an amount of flow of the gas supplied from the negative pressure applying device. Also, for the first reason, an amount of flow of the gas flowing through the pressure regulating device may become smaller than an amount of flow of the gas supplied from the negative pressure applying device.
As explained above, location of the pressure regulating device on the downstream side of the negative pressure applying device with respect to the purge direction may cause a portion of the purge passage on the downstream side to have a positive pressure more than necessary during the continued operation of the negative pressure applying device for any of the first to third reasons or any other reasons. In this case, it may be also possible to cause the pressure in a portion of the purge passage on the upstream side of the negative pressure applying device to be shifted to a positive pressure. Even in this case, the fuel vapor may forcibly flow from within the canister toward the intake pipe during the continued operation of the negative pressure applying device. However, a positive pressure may be applied to the canister after the negative pressure applying device is stopped. When this occurs, the gas contained within the canister may flow into the atmosphere passage resulting in the fuel vapor being discharged to the atmosphere via the atmosphere passage.
Therefore, there is a need in the art for preventing or inhibiting fuel vapor from flowing from a canister into the atmosphere after a negative pressure applying device is stopped.